Radio communication system

ABSTRACT

In a radio communication system having a primary station and a plurality of secondary stations, the power of uplink and downlink channels between the primary station and a secondary station is controlled in a closed loop manner by each station transmitting power control commands to the other station. In response to these commands the receiving station adjusts its output power in steps.  
     By considering a plurality of received power control commands the receiving station may emulate the ability to use power control step sizes other than those it directly implements, for example step sizes smaller than its minimum or intermediate between implemented step sizes. Performance can thereby be improved under certain channel conditions. In one embodiment when the required power control step size is less than the minimum step size of the receiving station, that station processes a group of power control commands to determine whether to adjust its output power by its minimum step size.

[0001] The present invention relates to a radio communication system andfurther relates to primary and secondary stations for use in such asystem and to a method of operating such a system. While the presentspecification describes a system with particular reference to theemerging Universal Mobile Telecommunication System (UMTS), it is to beunderstood that such techniques are equally applicable to use in othermobile radio systems.

[0002] There are two basic types of communication required between aBase Station (BS) and a Mobile Station (MS) in a radio communicationsystem. The first is user traffic, for example speech or packet data.The second is control information, required to set and monitor variousparameters of the transmission channel to enable the BS and MS toexchange the required user traffic.

[0003] In many communication systems one of the functions of the controlinformation is to enable power control. Power control of signalstransmitted to the BS from a MS is required so that the BS receivessignals from different MS at approximately the same power level, whileminimising the transmission power required by each MS. Power control ofsignals transmitted by the BS to a MS is required so that the MSreceives signals from the BS with a low error rate while minimisingtransmission power, to reduce interference with other cells and radiosystems. In a two-way radio communication system power control may beoperated in a closed or open loop manner. In a closed loop system the MSdetermines the required changes in the power of transmissions from theBS and signals these changes to the BS, and vice versa. In an open loopsystem, which may be used in a TDD system, the MS measures the receivedsignal from the BS and uses this measurement to determine the requiredchanges in the MS transmission power.

[0004] An example of a combined time and frequency division multipleaccess system employing power control is the Global System for Mobilecommunication (GSM), where the transmission power of both BS and MStransmitters is controlled in steps of 2 dB. Similarly, implementationof power control in a system employing spread spectrum Code DivisionMultiple Access (CDMA) techniques is disclosed in U.S. Pat. No.5,056,109.

[0005] In considering closed loop power control it can be shown that forany given channel conditions there is an optimum power control step sizewhich minimises the required E_(b)/N₀ (energy per bit/noise density).When the channel changes very slowly the optimum step size can be lessthan 1 dB, since such values are sufficient to track changes in thechannel while giving minimal tracking error. As the Doppler frequencyincreases, larger step sizes give better performance, with optimumvalues reaching more than 2 dB. However, as the Doppler frequency isfurther increased there comes a point where the latency (or update rate)of the power control loop becomes too great to track the channelproperly and the optimum step size reduces again, perhaps to less than0.5 dB. This is because the fast channel changes cannot be tracked soall that is needed is the ability to follow shadowing, which istypically a slow process.

[0006] Because the optimum power control step size can changedynamically it may improve performance if the BS determines anappropriate power control step size for use in uplink transmissions fromMS to BS and downlink transmissions from BS to MS, and informs the MSaccordingly. An example of a system which may use such a method is theUMTS Frequency Division Duplex (FDD) standard, where power control isimportant because of the use of CDMA techniques. Although improvedperformance can be obtained by having a small minimum step size, forexample 0.25 dB, this will significantly increase the cost of a station.However, if a station does not have to implement the minimum step sizethen it may not be able to implement the requested step size.

[0007] A further problem may occur in a system in which implementationof some power control step sizes by a station is optional. For example,in a system operating according to the UMTS specification a BS may use aplurality of different power control step sizes when changing thedownlink transmission power, for example the four step sizes 0.5 dB, 1dB, 1.5 dB and 2 dB. However, it may be the case that onlyimplementation of a 1 dB step size is mandatory. In some circumstancesit may be desirable to ensure that different BSs behave in a similarway. For example, during soft handover, a MS engages in communicationwith a plurality of BSs (known as the “active set” of BSs) to determineto which BS, if any, it should transfer. It is therefore necessary toavoid the transmission power of the BSs in the active set from divergingsignificantly. This is best achieved if the BSs in the active set changetheir transmission power in similar ways, for example by using similarpower step sizes in response to received power control commands.

[0008] If two BSs are in soft handover with a MS which is moving at aspeed such that 1.5 dB step sizes are optimal, but only one of the BSssupports 1.5 dB steps, optimum power control of both BSs is notpossible. The network has to choose between instructing the BSs to usedifferent step sizes, so that the optimum step size can be used by theBS supporting it (with the risk that the transmit powers of the two BSswill diverge significantly), or instructing both BSs to use the samenon-optimal step size (e.g. 1 dB or 2 dB) in order to avoid unduedivergence of transmit power. Clearly, neither choice is optimal.

[0009] An object of the present invention is to enable selection ofoptimum power control step sizes without requiring all stations toimplement the same set of step sizes.

[0010] According to a first aspect of the present invention there isprovided a radio communication system having a communication channelbetween a primary station and a secondary station, one of the primaryand secondary stations (the transmitting station) having means fortransmitting power control commands to the other station (the receivingstation) to instruct it to adjust its output transmission power, whereinthe receiving station has emulation means for emulating an unsupportedpower control step size by a combination of power control steps of atleast one supported size.

[0011] According to a second aspect of the present invention there isprovided a primary station for use in a radio communication systemhaving a communication channel between the primary station and asecondary station, the primary station having means for adjusting itsoutput transmission power in steps in response to power control commandstransmitted by the secondary station, wherein emulation means areprovided for emulating an unsupported power control step size by acombination of power control steps of at least one supported size.

[0012] According to a third aspect of the present invention there isprovided a secondary station for use in a radio communication systemhaving a communication channel between the secondary station and aprimary station, the secondary station having means for adjusting itsoutput transmission power in steps in response to power control commandstransmitted by the primary station, wherein emulation means are providedfor emulating an unsupported power control step size by a combination ofpower control steps of at least one supported size.

[0013] According to a fourth aspect of the present invention there isprovided a method of operating a radio communication system having acommunication channel between the primary station and a secondarystation, the method comprising one of the primary and secondary stations(the transmitting station) transmitting power control commands to theother station (the receiving station) to instruct it to adjust its powerin steps, wherein the receiving station emulates an unsupported powercontrol step size by a combination of power control steps of at leastone supported size.

[0014] The present invention is based upon the recognition, not presentin the prior art, that emulation of small power control step sizes by astation can provide good performance.

[0015] Embodiments of the present invention will now be described, byway of example, with reference to the accompanying drawings, wherein:

[0016]FIG. 1 is a block schematic diagram of a radio communicationsystem;

[0017]FIG. 2 is a flow chart illustrating a method in accordance withthe present invention for performing power control in a secondarystation;

[0018]FIG. 3 is a graph of the received E_(b)/N₀ in dB required for abit error rate of 0.01 against the power control step size used in dBfor a MS moving at 300 km per hour; and

[0019]FIG. 4 is a graph of the received E_(b)/N₀ in dB required for abit error rate of 0.01 against the power control step size used in dBfor a MS moving at 1 km per hour.

[0020] Referring to FIG. 1, a radio communication system which canoperate in a frequency division duplex or time division duplex modecomprises a primary station (BS) 100 and a plurality of secondarystations (MS) 110. The BS 100 comprises a microcontroller (μC) 102,transceiver means (Tx/Rx) 104 connected to radio transmission means 106,power control means (PC) 107 for altering the transmitted power level,and connection means 108 for connection to the PSTN or other suitablenetwork. Each MS 110 comprises a microcontroller (μC) 112, transceivermeans (Tx/Rx) 114 connected to radio transmission means 116, and powercontrol means (PC) 118 for altering the transmitted power level.Communication from BS 100 to MS 110 takes place on a downlink channel122, while communication from MS 110 to BS 100 takes place on an uplinkchannel 124.

[0021] In a UMTS FDD system data is transmitted in 10 ms frames eachhaving 15 time slots. The BS 100 transmits one power control command(consisting of two bits) per slot, where bits 11 (referred tohereinafter for simplicity as a value of 1) requests the MS 110 toincrease its power and bits 00 (referred to hereinafter as 0) requeststhe MS 110 to decrease its power. Changes in the required power controlstep size are notified separately over a control channel.

[0022] In a system according to the present invention this behaviour ismodified when the MS 110 is requested to implement a power control stepsize smaller than the smallest of which it is capable. In this situationthe MS 110 takes no action unless it receives a series of identicalpower control commands, thereby emulating the performance of a MS 110having more precise power control.

[0023] Consider for example the case where the requested step size is0.5 dB and the minimum step size implemented by the MS 110 is 1 dB. TheMS 110 processes power control commands in pairs and only changes itsoutput power if both commands are equal. Hence if the received commandsare 11 the power is increased, if they are 00 the power is decreased,and if they are either 10 or 01 the power is not changed. It may beadvantageous to align the comparison with the transmission of frames,hence to combine the power control commands transmitted in slots 1 and 2of a particular frame, then the commands transmitted in slots 3 and 4,and so on.

[0024] Similarly, if the requested step size is 0.25 dB and the minimumstep size is 1 dB the MS 110 processes power control commands four at atime, and only changes its output power if all four commands are equal.Hence the power is increased if the received commands are 1111,decreased if they are 0000, and unchanged otherwise. Again it may beadvantageous to align the comparison with the frame transmission,combining the commands transmitted in slots 1 to 4 of a particularframe, then the commands transmitted in slots 5 to 8 and so on.

[0025] Combining the commands received in three or five slots isparticularly advantageous in the UMTS embodiment being consideredbecause it maintains alignment with a frame of 15 slots. However, themethod is not restricted to such a system. Consider a general case wherethe minimum step size implemented by the MS 110 is S and the step sizerequested by the BS 100 is R. In this case the power control commandsmay be combined in groups of G, where G=S/R.

[0026]FIG. 2 illustrates a method of emulating smaller power controlsteps than the minimum of the MS 110. The method starts, at 202, withthe MS 110 determining G, the number of commands to be combined in agroup and setting a received power control command counter i to zero. At204 the MS 110 receives a power control command and increments thecounter i. Next, at 206, the value of i is compared with G. If i is lessthan G then the received command is stored and the MS 110 waits toreceive the next command. Otherwise the required number of power controlcommands have been received and the MS 110 determines, at 208, if itshould adjust its power based on the received power control commands.Once this has been done the counter i is reset to zero (if i is equal toG) or to one (if i is greater than G, which will happen if G is notinteger) and the MS 110 waits to receive the next power control command.

[0027] In an alternative embodiment, instead of combining power controlcommands in groups of G the MS 110 keeps a running total of therequested power change and makes a change once the total requested powerchange reaches its minimum step size. For example, if the requested stepsize is 0.25 dB and the minimum step size is 1 dB the sequence ofreceived commands 11010111 would result in the power being increased by1 dB. The MS 110 then subtracts the step actually implemented from therunning total of the requested power change. However, such a scheme ismore complex to implement (since it requires maintaining a running totalof the requested power change) and it appears to provide only a minimalimprovement to the performance of the method.

[0028] In a variation of this alternative embodiment, the MS 110 uses asoft decision method in keeping a running total of the requested powerchange, instead of taking a hard decision on each individual powercontrol command. Each power control command is weighted by a function ofthe amplitude of the received signal for that command, as a measure ofthe likelihood of the MS 110 having correctly interpreted the command,before being added to the running sum. For example, the sequence11010111011 might, once weighted, correspond to the sequence ofrequested power changes 0.8 0.3-0.3 0.4-0.1 0.5 0.9 0.8-0.4 0.7 0.5 (inunits of 0.25 dB). This sequence has a running sum of 4.1 which wouldtrigger the MS 110 to execute an upwards step of 1 dB and to reduce therunning sum to 0.1. This variation should provide a slight improvementin the performance of the method.

[0029] Two simulations have been carried out to illustrate theeffectiveness of the method according to the present invention. Theseexamine the performance of a MS 110 having a minimum step size of 1 dBcompared with that of a MS 110 having a minimum step size of 0.25 dB.The simulations make a number of idealising assumptions:

[0030] there is a 1 slot delay in the power control loop;

[0031] there is no channel coding;

[0032] there is perfect channel estimation by the receiver;

[0033] equalisation in the receiver is carried out by a perfect RAKEreceiver;

[0034] no control channel overhead is included in the E_(b)/N₀ figures;there is a fixed error rate in the transmission of power controlcommands; and

[0035] the channel is modelled as a simple N-path Rayleigh channel.

[0036] The first simulation relates to a rapidly changing channel, witha MS 110 moving at 300 km per hour in a single path Rayleigh channelwith an error rate for the power control commands of 0.01. FIG. 3 is agraph of the received E_(b)/N₀ in dB required for an uplink bit errorrate of 0.01 against the power control step size used in dB. The solidline indicates results for a MS 110 having a minimum power control stepsize of 0.25 dB or less, while the dashed line indicates results for aMS 110 having a minimum step size of 1 dB which combines power controlbits in groups of two or four to emulate 0.5 dB and 0.25 dB powercontrol step sizes respectively.

[0037] In this situation the best performance is obtained for small stepsizes of less than 1 dB. Emulation of 0.25 dB and 0.5 dB steps resultsin a small implementation loss of only about 0.05 dB, compared to about0.6 dB if no emulation is performed, demonstrating the usefulness of theemulation method. Increasing the error rate of the power controlcommands to 0.1 produces a general degradation of about 0.2 dB in thereceived E_(b)/N₀, but the performance of the MS 110 with emulated smallsteps remains close to that of the MS 110 with direct implementation ofsmall steps.

[0038] The second simulation relates to a slowly changing channel, witha MS 110 moving at 1 km per hour in a six path Rayleigh channel with anerror rate for the power control commands of 0.01. FIG. 4 is a graph ofreceived E_(b)/N₀ in dB required for a uplink bit error rate of 0.01against the power control step size used in dB. The lines in the graphare identified in the same way as for FIG. 3.

[0039] In this situation there is a small advantage in using powercontrol steps of less than 1 dB. As with the first simulation, theresults obtained using emulated small steps are very close to those withdirect implementation of small steps.

[0040] In a further application of this method the value of G may be setto a value other than S/R if it is considered to be advantageous forreasons such as reducing the effect of errors in the interpretation ofthe transmitted power control commands (for example by averaging over agreater time period). In some circumstances a MS 110 might thereforechoose to use a step size larger than the minimum which it is capable ofimplementing.

[0041] A variation of the method described above can be employed for theemulation of unsupported power control step sizes greater than theminimum step size implemented by a station. Consider the case of a BS100 in a system operating according to the UMTS specification. In oneexample of such a system the BS 100 may use one of four step sizes whenadjusting the power of the downlink transmission 122, namely 0.5 dB, 1dB, 1.5 dB and 2 dB, of which only 1 dB is mandatory.

[0042] Consider the situation where the BS 100 is instructed by thenetwork infrastructure to use 1.5 dB steps but only implements 1 dB and2 dB steps. In a method in accordance with the present invention the BS100 considers the received power control commands in pairs. For useduring soft handover it is advantageous for these groups to be alignedto either an odd- or even-numbered frame boundary, since a frameincludes an odd number (15) of timeslots. The definition of an even orodd frame can be determined from a connection frame number or systemframe number. Such alignment ensures that different BSs 100 in theactive set, which are executing an emulation algorithm in accordancewith the present invention, behave in a similar way.

[0043] In the first timeslot of each pair the BS 100 always implements apower step of 1 dB in the direction given by the sign of the receivedpower control command, where the sign is considered to be negative ifthe received command is 0 and positive if the received command is 1. Inthe second timeslot, the BS 100 implements a power step of 2 dB if thereceived power control command is of the same sign as that received inthe first slot, or a power step of magnitude 1 dB if the signs areopposite. If a BS 100 only implements 1 dB steps, more than one 1 dBstep could be performed in a single timeslot if a larger step size isrequired by the emulation algorithm. The resultant power changes are:commands power change 1^(st) slot 2^(nd) slot 1^(st) slot 2^(nd) slot 00 −1dB −2dB 0 1 −1dB +1dB 1 0 +1dB −1dB 1 1 +1dB +2dB

[0044] The above method can be generalised to handle the case ofemulating step sizes equal to (x+0.5) dB where the BS 100 can implementsteps of x dB and (x+1) dB by having the power step in the firsttimeslot of x dB and the power step in the second timeslot of x dB or(x+1)dB as appropriate.

[0045] Further generalisation is also possible. Consider the case ofemulating step sizes equal to (x+a)Δ dB, where Δ is the smallest stepsize supported by the BS 100, x is an integer and 0<a <1. Each time thatthe BS 100 receives a power control command it performs the followingcalculation:

S _(i) =S _(i−1) +Pa

[0046] where P is equal to −1 when the received command has a value of 0and is equal to +1 when the received command has a value of 1. S_(i−1)is initialised to zero in the first timeslot, and thereafter is equal tothe value of S_(i) in the previous timeslot.

[0047] If S_(i)≦0.5, the size of the power step implemented by the BS100 is xΔ dB. If S_(i)>0.5, the size of the power step implemented bythe BS 100 is (x+1)Δ dB, and the BS 100 subtracts P from S_(i).

[0048] Now consider the case of emulating step sizes equal to (x+a/b)ΔdB, where x, a and b are integers and a<b. The BS 100 considers receivedpower control commands in groups of b. For the soft handover case it ispreferred that the groups are aligned to an odd- or even-numbered frameboundary, for the same reasons as given above for the basic emulationalgorithm.

[0049] The BS 100 divides the group of b timeslots into a sub-groups,such that there is at most a difference of 1 in the number of timeslotsin each subgroup. In all timeslots except the last one in eachsub-group, the BS 100 always implements a power step of magnitude xΔ dBin the direction given by the sign of the received power control commandin that slot. In the last slot of each sub-group, the BS 100 implementsa power step of magnitude (x+1)Δ dB if the received power controlcommands in all slots of that sub-group are of the same sign, otherwiseit implements a power step of magnitude xΔ dB. This method ensures thatthe error in power level is never greater than the greater of a/b dB and(1−a/b) dB.

[0050] The methods described above may also be further generalised toinclude the emulation of any step size intermediate between two stepsizes supported by a BS 100 or MS 110.

[0051] In the description above, any reference to emulation of stepsizes by a MS 110 for controlling the power of the uplink transmission124 could equally well be employed by a BS 100 for controlling the powerof the downlink transmission 122, and vice versa.

[0052] Further, the detailed description above relates to a system wherepower control commands are transmitted separately from instructions to astation to set its power control step size. However, the presentinvention is suited for use in a range of other systems. In particular,it can be used in any system in which there is a variable power controlstep size and in which a station is instructed to use a particular valuefor this step. It can also be used in systems in which the power controlstep size is fixed, or at least fixed while a power control step sizeemulation method is being used. The particular step size to be used by astation could be determined by the network infrastructure, the BS 100,or the MS 110. It could also be determined by negotiation between any ofthese entities.

[0053] From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in radio communication systems,and which may be used instead of or in addition to features alreadydescribed herein.

[0054] In the present specification and claims the word “a” or “an”preceding an element does not exclude the presence of a plurality ofsuch elements. Further, the word “comprising” does not exclude thepresence of other elements or steps than those listed.

1. A radio communication system having a communication channel between aprimary station and a secondary station, one of the primary andsecondary stations (the transmitting station) having means fortransmitting power control commands to the other station (the receivingstation) to instruct it to adjust its output transmission power, whereinthe receiving station has emulation means for emulating an unsupportedpower control step size by a combination of power control steps of atleast one supported size.
 2. A primary station for use in a radiocommunication system having a communication channel between the primarystation and a secondary station, the primary station having means foradjusting its output transmission power in steps in response to powercontrol commands transmitted by the secondary station, wherein emulationmeans are provided for emulating an unsupported power control step sizeby a combination of power control steps of at least one supported size.3. A primary station as claimed in claim 2, characterised in that theemulation means emulates a step size of (x+0.5)Δ dB, where x is aninteger and Δ is the minimum step size implemented by the station, theemulation means considering received power control commands in pairs andimplementing a step of magnitude xΔ dB in response to the first powercontrol command followed by a step of magnitude (x+1)Δ dB if the secondpower control command has the same sign as the first, and by a step ofxΔ dB if the power control commands are of opposite sign.
 4. A secondarystation for use in a radio communication system having a communicationchannel between the secondary station and a primary station, thesecondary station having means for adjusting its output transmissionpower in steps in response to power control commands transmitted by theprimary station, wherein emulation means are provided for emulating anunsupported power control step size by a combination of power controlsteps of at least one supported size.
 5. A secondary station as claimedin claim 4, characterised in that the emulation means processes aplurality of power control commands as a group to determine whether toadjust its output power, thereby emulating a power control step sizesmaller than the minimum step size.
 6. A method of operating a radiocommunication system having a communication channel between the primarystation and a secondary station, the method comprising one of theprimary and secondary stations (the transmitting station) transmittingpower control commands to the other station (the receiving station) toinstruct it to adjust its power in steps, wherein the receiving stationemulates an unsupported power control step size by a combination ofpower control steps of at least one supported size.
 7. A method asclaimed in claim 6, characterised by emulation of a step size of(x+0.5)Δ dB, where x is an integer and Δ is the minimum step sizeimplemented by the receiving station, the emulation being performed byconsidering received power control commands in pairs and by implementinga step of magnitude xΔ dB in response to the first power control commandfollowed by a step of magnitude (x+1)ΔdB if the second power controlcommand has the same sign as the first, and by a step of xΔ dB if thepower control commands are of opposite sign.
 8. A method as claimed inclaim 7, characterised by transmissions on the channel taking place inframes and by the pairs of power control commands being aligned withrespect to the start of an even-numbered frame.
 9. A method as claimedin claim 7, characterised by transmissions on the channel taking placein frames and by the pairs of power control commands being aligned withrespect to the start of an odd-numbered frame.
 10. A method as claimedin claim 6, characterised by emulation of a step size of (x+a)Δ dB,where x is an integer, a is between 0 and 1, and Δ is the minimum stepsize implemented by the receiving station, the emulation being performedby maintaining a running sum of the difference between the sum of therequested power control steps and the sum of the implemented powercontrol steps and by implementing a step of magnitude (x+1)Δ dB if thedifference between the sums is greater than 0.5Δ and a step of magnitudexΔ dB otherwise.
 11. A method as claimed in claim 6, characterised bythe receiving station processing a plurality of power control commandsas a group to determine whether to adjust its output power, therebyemulating a power control step size smaller than its minimum step size.12. A method as claimed in claim 11, characterised by transmissions onthe channel taking place in frames and by the groups of power controlcommands having predetermined positions with respect to the start ofeach frame.
 13. A method as claimed in claim 12, characterised by thesize of the group being exactly divisible into the number of powercontrol commands transmitted in a frame.